Liquid storage tanks are commonly used in petroleum production and at industrial facilities. These tanks are used to store petroleum products, contaminated wastewater, or process chemicals. These materials may contain flammable, volatile components that present an explosion hazard. If a tank contains flammable vapors and air, an electrostatic discharge can trigger a dangerous and costly explosion.
Consequently, electrostatic drain devices are sometimes employed inside storage tanks. The electrostatic drain device safely discharges electrostatic charges in the contained air and liquid to ground potential, thereby eliminating the possibility of an electrostatic explosion trigger.
FIG. 1 shows a conventional storage tank with electrostatic drain according to the prior art. The tank 10 contains a liquid and a mixture of air and explosive vapors. The explosive vapors may comprise low molecular weight hydrocarbon vapors such as butane for example. The liquid flows into and out of the tank 10 via a pipe connection 11. As the liquid moves through the pipe, electrostatic charge is created in the liquid via well-known triboelectric effects. This electrostatic charge will become trapped in the tank if there is no conductive path to ground potential. The trapped electrostatic charge can trigger an explosion of the air and flammable vapor mixture.
Nonconductive tanks (e.g. made of polymers or fiberglass) are particularly problematic because they do not provide an electrically conductive path to ground potential. Metal tanks can also present a hazard if they are coated with an electrically insulating coating of epoxy or paint.
The prior art solution to this problem is to use a metal twisted wire brush 12 as an electrostatic drain. The metal wire brush device 12 is suspended inside the tank 10 and electrically connected to ground potential 13. The wire brush comprises a twisted cable 14 with embedded small diameter wires 15 (e.g. 0.001-0.020″ diameter). The small diameter wires have sharp tips that serve to concentrate an electric field, and thereby facilitate charge collection. The wire brush 12 is typically made entirely of stainless steel. In operation, the drain device accumulates electrostatic charge present in the liquid and air, and provides a path for this charge to flow to ground potential 13.
The conventional solution of FIG. 1 is effective for dissipating electrostatic charge. However it has several serious disadvantages, including cost, susceptibility to corrosion, difficulty of installation (since the central twisted wire is rigid or semi-rigid), and tendency of the small wires to loosen and fall off over time. The small wires can loosen because they are held at only a single point where they pass through the twisted cable. Hence if corrosion causes one wire to dislodge, then all other wires in the same bundle will fall out as well. Small wires or corroded metal particles that fall into the liquid will damage downstream equipment Consequently, the wire brush 12 presents a significant hazard for liquid-handling equipment such as filters, valves and pumps.
Corrosion is a great concern at petroleum facilities because the liquids in the tank often contain combinations of salts, acids, hydrogen sulphide and other substances that corrode many types of metals, including stainless steel. This is one reason why non-metallic tanks are preferred for these applications.
Fiberglass tanks are corrosion resistant, but because they are electrically insulating, fiberglass tanks are an explosion hazard.
FIG. 2 shows a conventional method for fabricating a composite-fiberglass tank. A cylindrical mandrel 16 is used as a form that defines the internal shape and dimensions of the tank. A fiberglass strip 17 is soaked in curable resin (e.g. polyester resin) and wrapped around the mandrel. The fiberglass strip can also be wrapped in a diagonal or zigzag fashion. After the mandrel is covered with resin-soaked fiberglass, the resin is cured, and the resulting tube is removed from the mandrel. Bottom and top covers (not shown) are attached to enclose the ends of the tube. A problem with this method is that all inside surfaces of the tank are electrically non-conductive due to a thin layer of non-conductive resin coating the inside surfaces. Even when conductive carbon fiber is used instead of fiberglass, the inside surface of the tank is electrically non-conductive due to a layer of resin.
It would be a great advantage and improvement in the art to provide a composite fiberglass tank with integral electrostatic charge dissipation functionality.